The subject invention is directed to a battery interseparator suitable for use in an alkaline battery system and to a method of forming the same.
Because of their high energy density, alkaline battery systems, such as nickel-zinc secondary alkaline battery systems, have great potential for replacing the more conventional lead-acid battery system in a number of terrestrial applications. However, extending the cyclic life of such batteries beyond that presently attainable and reducing the cost of all the components are required criterias which must be met to make the alkaline battery system an effective energy source.
One of the recognized key components in attaining an extended battery life and efficiency is the battery interseparator. The interseparator is a porous diaphragm located between the positive and/or negative plates and the dendristatic separator diaphragm of an alkaline battery system so as to (1) provide an electrolyte reservoir, (2) provide uniform electrolyte distribution across electrode and separator surfaces to permit uniform current density and, (3) allow space for electrode expansion during use. In order to achieve these results, the resultant diaphragm must be capable of exhibiting a high degree of absorption or wicking, and be sufficiently porous to carry and evenly distribute the electrolyte of the battery system.
It is also desirable to have an interseparator diaphragm which is very thin, such as less than 10 mils, in order to minimize the amount of electrolyte required and, thereby, maximize the energy density of the resultant system. It has not been generally believed that one can produce a sheet product of such small thicknesses due to the high content and nature of the filler required in interseparator sheets suitable for alkaline battery systems.
Conventional lead-acid and secondary alkaline battery systems have certain components, such as electrodes, electrolytes, separators, etc., which, though commonly named, are distinctly different entities having different functions, and are required to have different physical and chemical properties. It is easily recognized that the electrodes of a lead-acid battery system are distinctly different from the electrodes used in a secondary alkaline battery system, such as a nickel-zinc alkaline battery system. Similarly, separators used in a lead-acid system are distinctly different from those used in a secondary alkaline battery system. The lead-acid battery separator is a material positioned between electrode plates of opposite polarity t insure maintenance of a separation. Any material which is superimposed between adjacent surfaces of the plates to retain the desired separation is satisfactory. Normally, these separators are produced of materials which are capable of being formed into sheets of (1) substantial thickness or with a matte surface to aid in the separation of the plates, (2) substantial porosity to readily permit the electrolyte to pass through, and (3) must be chemically inert to acidic electrolyte. Separators of alkaline battery systems not only aid in separating the plates of opposite polarity, but mainly function as a dendristatic diaphragm. The separator of alkaline battery systems, such as a nickel-zinc secondary alkaline battery, must, therefore, have very low porosity in order to inhibit the growth of dendrites therethrough, must be very thin to minimize electrical resistance, and must be of a material which is chemically inert to alkaline electrolyte while allowing electrolyte passage therethrough.
Alkaline battery systems usually utilize an interseparator in combination with the dendristatic separator diaphragm. This is especially true with alkaline battery systems which utilize nickel and/or zinc electrodes. The interseparator, as discussed above, must have a combination of properties including having a high degree of wicking capability, such as at least about 5 cm/24 hours as determined by standard techniques, to aid in overcoming the deficiencies of the separator diaphragm, and be formed of material which is capable of being produced into very thin, highly porous sheets of good integrity. The interseparator component is specific to alkaline battery systems.
Battery interseparators which are used today in alkaline battery systems are commonly composed of polypropylene, polyamide, or nylon non-woven sheets. These interseparators suffer from sufficient wicking and/or lack the necessary chemical and/or oxidation resistance in an alkaline environment to effectively aid in enhancing the battery system. The development of alkaline secondary batteries, particularly nickel-zinc, has been hampered by the absence of suitable interseparators for these applications.
It is an object of the present invention to provide a fibrous, filled battery interseparator suitable for use in an alkaline battery system.
It is another object of the present invention to provide a battery interseparator that can be prepared on a conventional paper machine, has a significantly smaller maximum pore-size than conventional non-woven fiber-based battery interseparator webs, and, thereby, aids in inhibiting dendrite growth along with the separator used in conjunction therewith.
It is still another object of the present invention to provide a battery interseparator of a thinness of not greater than 10 mils which can be prepared on a paper machine and exhibits a tensile strength of greater than 200 lbs/in.sup.2 in the machine direction, and sufficient flexibility to be formed around the electrode plate.